Contemporary cellular radio networks are known since many years now and are based on different technologies. The broadest coverage still is held by the global system for mobile communications according to the so called GSM standard. A user equipment in such cellular network can move freely and may be handled over to various cells of the GSM networks as for instance described in GSM standard specification 3GPP ETSI TS 51.010 or the like.
Contemporary radio networks are based on a cellular code division multiple access (CDMA) as for instance realized in the universal mobile telecommunication system (UMTS). The latter is increasingly important for security applications like camera systems or the like.
Generally, a user equipment in radio networks can be subject of being affected by a jamming transmitter—jamming in this context generally is performed by an instrument preventing a user equipment from receiving signals from its base station. In use the jammer effectively disables cellular phones mostly by broad frequency interference with communication frequencies of the user equipment at high power level. Whereas some jammer applications are meant to be legal for instances in places where a phone call is to be suppressed due to silence conditions, other jammers are applied during misuse for instances to interrupt security applications of user equipment or the like. Jammers are available for jamming GSM and also UMTS frequencies. However, jamming detecting and preventing solutions are known up to date basically only against GSM jammers. In this regard, it should be recognized that the primary aim of an anti-jamming solution is to undoubtfully detect a jamming attack rather than preventing the same.
An anti-jamming solution is known from WO 2007/019814 which however is restricted to the GSM standard. Therein a method for detecting a jamming transmitter affecting a communication terminal is described wherein receipt of radio channel signal levels are evaluated at periodic intervals on a signalling channel. In the case that the communication terminal detects a radio channel signal level that exceeds a predefined threshold value in the signalling channel but is nevertheless unable to decode a message content of a message, then this state is interpreted as an interference state and an alarm signal is emitted. The problem related with this GMS anti-jamming solution is its fundament on a predefined threshold value in the signalling channel and the receipt of a message content. These features are somewhat specific for the GMS technology, however, less suited in the UMTS technology. More specifically it turns out that an anti jamming solution in the frame of a cellular code division multiple access based radio network is much more demanding. The state of dealing with disturbances in a communication frequency band of a user equipment is more or less a usual state of operation for a user equipment within a cellular code division multiple access based radio network. In particular, intracell and intercell interferences are generally accepted in a CDMA based radio network as long as a signal can be decoded. Thus, the state of operation naturally is permanently disturbed due to the CDMA based technology. A discrimination of a severe jamming action among these natural disturbances is still a problem to be solved.
The specific reason is as follows. A communication user equipment (UE) and a number of base node stations (BNS) are the basic components of a CDMA based radio network. The radio network (RN) may work in either a frequency division duplex (FDD) or also a time division duplex (TDD) mode. Once a communication link in a serving cell coverage area is provided between the communication user equipment and a serving base node station (sBNS) a communication signal unit (SU) is correlated with a pseudonoise spread code (SC) in a serving cell coverage area (CA) of a serving base node station and transmitted as a pseudonoise chip (CHI) in a multiple shared communication frequency channel. Thus, interferences of multiple base node stations and user equipments in the communication frequency channel are spectrally located between an upper frequency and a lower frequency of a communication frequency band. Consequently, a broad band “jamming like” interference in the multiple shared communication frequency channel can not be considered as an extraordinary event but is on the contrary part of the usual state of operation. Such situation may also occur each time the number of users changes in said frequency band. The similar situation may also occur when a user equipment has a comparatively large or a comparatively small distance to a base node station. Also a similar situation may occur when a user equipment is in the reach of two base node stations in particular vice versa when two user equipments belong to the same or neighbouring cells of the CDMA based radio network. In conclusion, an anti-jamming solution to be successfully implemented in a CDMA based radio network technology is more sophisticating.
In WO 00/62437 a concept for improving a jammer detection sensitivity in a CDMA based communication network is provided wherein a spectral analysis data is used to identify jamming signals having power spectral density characteristics which are distinguishable from those of legitimate subscriber transmissions in the wireless system's frequency band. By using several base stations located near the jamming transmitter, and by comparing the power spectral densities received at those base stations, the location of the jamming transmitter is estimated. Additionally, such spectral analysis data is used to detect aberrant receive spectrum characteristics which may indicate a hardware malfunction or failure. The spectral analysis uses a model of a real-input-data FFT and complex-input-data FFT for a CDMA signal bandwidth C of approximately 1.25 MHz and is based on the assumption that a jammer detection threshold will be set relative to a “noise floor”, and it can be concluded that the jammer detection threshold will be the same for the two cases of a FFT. The (in-band) power spectral density P will be the same for either technique, with the power spectral density equaling P/C. But because the jammer power divided equally between a I and a Q branch, the jammer power will be 3 dB less for the real-input-data FFT than in the case of the complex-input-data FFT.
Nevertheless, generally and as compared to the above mentioned GSM solution of WO 2007/019814, a predefined threshold value for a signal level of a specific signalizing channel for a user equipment per se cannot be defined. Either the channel and/or the signal level is continuously changing depending on the surroundings of the network. Also, a message content as such can not be received unless a pseudonoise spread code is received by the communication user equipment. Consequently, without pseudonoise spread code neither transmission nor a content of a message is possible unless—the pseudonoise spread code is known to the user equipment.
In 3GPP TS 25.133, in particular in Chapter 4.2.2.1, it is defined if a user equipment cannot find a suitable UTRA cell, then it is considered to be “out of service area” and shall perform actions according to 3GPP TS 25.331 (“RRC Protocol Specification”).